Photovoltaic (PV) cells are semi-conductor materials that generate an electric current when irradiated with an infra-red (IR) photon source. The required temperature of the PV cell's energy source, the emitter, must be high enough to emit significant radiation above the PV cell's characteristic bandgap energy. Photons energetic enough to induce the bandgap electron transitions in the PV material will generate an electric current. In general, the photon or radiant emission of a material increases with temperature. Therefore, higher emitter temperatures can substantially reduce the PV cell area required to generate a given level of power.
Solar photovoltaic technology utilizes relatively inexpensive Si cells in which the bandgap energy is 1.12 eV (1.11 microns). The sun is a high temperature source (6000.degree. K.) with much of its radiant emission above this bandgap energy. A solar cell is typically capable of generating only about 0.02 Watts/cm.sup.2 in direct sunlight. This is due to the sun's large distance from the earth which diminishes the useful energy flux. Therefore, solar photovoltaic power systems require relatively large solar panels to collect sufficient power for residential dwellings and are confined to locations and climates with sufficient sunlight. Reflective surfaces are often utilized in this technology to concentrate the sun's energy on the photovoltaic surface.
FIG. 1 illustrates a typical TPV system, generally designated 10. A fossil fuel 12 is combined with preheated combustion air 14 for combustion in a burner 18. Radiative and convective heat transfer 20 from the combustion process occurring at burner 18 elevates the temperature of an emitter 22. The emitter 22 then radiates energy 24 to photovoltaic (PV) cells 26 which convert incident radiant energy into electrical energy 28 in the form of a direct current or DC. However, because PV cells 26 cannot convert all wavelengths of light into electricity, optical filters 30 are used to filter out less useful wavelengths and permit desirable wavelengths 32 of radiant energy to strike the PV cells 26. This is important because that portion of the incident energy absorbed by the PV cells 26 which is not converted into electricity must be removed as waste heat 34. A recuperator 36 is used to boost the TPV system 10's efficiency by transferring a portion of the energy in hot combustion products 38 produced by burner 18 into incoming combustion air 14 to produce the preheated combustion air 16, and prior to exhaust 40 of the combustion products 38 into the atmosphere.
Development efforts on TPV technologies go back to the early 1960s. Recently renewed interest in TPV has occurred with the development of more efficient lower bandgap PV cell designs that can be coupled with lower temperature IR emitter sources. It is now in the realm of possibility to generate sufficiently high emitter temperatures with fossil fuel combustion, and to fabricate emitter materials that can withstand these temperatures. To use silicon cells efficiently would require emitter temperatures greater than 2000.degree. K. This high temperature would result in rapid degradation and failure of the emitter and other high temperature components of the system. However, the recent development of low bandgap semiconductor materials allow the use of lower emitter temperatures. The gallium antimonide (GaSb) PV cells described by Fraas et al. in "Fundamental Characterization Studies of GaSb Solar Cells" (22nd IEEE Photovoltaic Specialists Conference, IEEE, New York, 1991, pp. 80-89) are sensitive in the IR radiation out to 1.8 .mu.m (0.73 eV). See FIG. 2.
Currently, systems producing up to several kilowatts are under development. Commercial viability will be largely dependent on fabrication costs per unit of useable electrical power output. The low bandgap PV cells are expected to be the most costly component of the TPV electric generator. The quantity of PV cells can be minimized with the design of energy efficient systems, and with the development of emitter materials that can withstand higher temperatures to produce higher useful photon fluxes. According to Noreen and Du in "High Power Density Thermophotovoltaic Energy Conversion" (The First NREL Conference on Thermophotovoltaic Generation of Electricity, Copper Mountain, Colo., 1994, pp. 119-131), it was estimated that IR emitter sources need to produce a sufficient energy flux to generate approximately 7.5 to 10 Watts/cm.sup.2 to be commercially viable given current PV cell technology. This would allow fabrication of compact, lightweight power systems which can produce significant power at reasonable costs. These are only some of the technical challenges that must be surmounted to make TPV technology a cost effective and energy efficient alternative. Potential applications for small portable power generators include: the military, commercial customers in remote areas without easy access to a utility grid, cogeneration for residential and commercial dwellings, and self-powered appliances.
Several R & D programs are under way to develop TPV power system components. Much effort has gone into fabricating durable, selective emitters that efficiently couple their radiance to the PV cell's bandgap energy. The tradeoff for higher energy efficiency utilizing selective emitters is lower useful photon fluxes (power density). Broadband (black body) emitters increase the photon convertible radiance on the emitter which necessitates less PV cell area for a given power output. This significantly reduces system cost. However, PV cell conversion efficiency suffers. Furthermore, out of band photon energy is also significantly higher with broadband emitters, and this radiant energy must be recycled back to the emitter with elaborate spectral filters and reflectors to maintain high system efficiency. The debate between broadband and selective emitters as a system choice continues. In the near term, and as discussed in the aforementioned Noreen and Du reference, some investigators feel that broadband emitters may be more appropriate for the larger scale TPV systems that approach a kilowatt. The higher power densities will be necessary to keep costs down by minimizing the number of PV cells.
Selective emitter materials tend to have low thermal conductivities and are susceptible to thermal stresses. For this reason much of the selective emitter development has taken the approach of fabricating fibrous porous emitters on which a fossil fuel flame is stabilized. This design approach provides excellent thermal coupling with the flame. As a result, emitter materials quickly approach the flame temperature despite the low thermal conductivity. Fibers can also bend and thus thermal stresses can be relieved. However, mechanical durability is poor. See for example, Nelson, "Thermophotovoltaic Emitter Development" (The First NREL Conference on Thermophotovoltaic Generation of Electricity, Copper Mountain, Colo., 1994, pp. 80-95) and Holmquist et al., "Laboratory Development TPV Generator" (The Second NREL Conference on Thermophotovoltaic Generation of Electricity, Colorado Springs, Colo., 1995, pp. 138-161) which describe development efforts for these selective emitters, and which include Supported Continuous Fiber Radiant Structures (SCFRS) in which a solid ceramic support structure is fabricated with an array of holes in which small fiber bundles are mounted with a ceramic epoxy. This design replaces a single large fibrous structure with several small fibrous bundles and improves strength. Other work includes flow through ceramic felts utilizing a replication process on a rayon felt mat. Both of these approaches use Yb.sub.2 O.sub.3 as the selective emitter material, and seek to improve spectral properties and increase resistance to mechanical and thermal stresses. Both of these approaches require a transparent window between the emitter and the PV cells to isolate them from the high temperature products of combustion. This window must have high temperature capabilities and/or be cooled. The most economical choice for this material is quartz; however, with high emitter temperatures, quartz windows must be cooled which significantly complicates the system and reduces efficiency.
Another approach for the emitter uses solid monolithic or composite structures where the emitting surface forms a physical barrier between the PV cells and hot combustion gases. This approach usually utilizes a broadband emitter such as SiC. SiC ceramic tubes have undergone much development work as IR heat sources for industrial heating applications. See for example, Singh "Design of a High Temperature Gas-Fired Heating System Utilizing Ceramics" (Industrial Heating, November 1988, pp. 18-20) and Vinton "Ceramic Radiant Tube System Speeds Batch Furnace Recovery" (Heat Treating, February 1989, pp. 24-27). SiC ceramic tubes are excellent gray body emitters with emittance values close to 0.9 over a significant energy range including the IR. SiC also has a high thermal conductivity. Therefore, SiC ceramic tube emitters exhibit excellent thermal shock resistance, and the thermal resistance through the monolithic layer to the emitting surface is minimized. The maximum operating temperature of SiC is approximately 2000.degree. K. As described in Pernisz, et al. "Silicon Carbide Emitter and Burner Elements for a TPV Converter" (The First NREL Conference on Thermophotovoltaic Generation of Electricity, Copper Mountain, Colo., 1994, pp. 99-105), some investigators are developing innovative approaches to the fabrication of SiC burner elements for TPV applications. Utilizing organic siloxanes as precursors, additional additives, and careful control over the pyrolysis step, these investigators have achieved considerable flexibility in the physical characteristics of SiC forms. With the ability to control density and porosity, and high emittance (.gtoreq.0.84), SiC was deemed an excellent material candidate for broadband emitting burner elements.
Conceptual designs for systems utilizing broadband and selective emitters have been patented. See, for example, U.S. Pat. No. 4,836,862 to Pelka et al., drawn to a combustor/reactor for a TPV process employing recuperation of energy from the products of combustion.
In Saraf et al., "Design of a TPV Generator with a Durable Selective Emitter and Spectrally Matched PV Cells" (The Second NREL Conference on Thermophotovoltaic Generation of Electricity, Colorado Springs, Colo., 1995, pp. 98-108), a TPV generator design is proposed that operates at 1100.degree. C. to generate 250 watts of useable electric power, A cylindrical emitter, 6 inches in diameter and 10 inches long, emits inward to a 2 inch cylinder that supports six (6) separate 1/2 inch wide InGaAs PV strips with a bandgap of 0.6 eV. Inside the cylindrical PV cell support, a porous metal heat exchanger utilizes water at one (1) liter per minute to cool the PV cells. Parabolic reflectors focus the emitter radiance on the PV cell strips. A selective holmia emitter that is of a durable thermal shock resistant design would be used, and electric power output densities of 1.5 to 1.75 W/cm.sup.2 were anticipated.
Laboratory work has also been performed in the development of conceptual designs for TPV electric generators. As described above, Nelson has been leading work in the development of a gas fired system utilizing SCFRS. As published by Coutts et al. in "A Review of Recent Advances in Thermophotovoltaics" (The 25th IEEE Conference, Washington, D.C., May 1996), these designs have a useable radiance of 4 W/cm.sup.2 for 160 W/cm.sup.2 of fuel input, and that usable radiance values of even 6 W/cm.sup.2 has been achieved.
As described by Holmquist et al., supra, a methane/oxygen fired TPV generator is being developed that produced 2.4 kW at a claimed efficiency of 4.5%. The design utilized a flow-through selective emitter fabricated from a ceramic oxide (ytterbia). The process utilized a replication process to form a selectively emitting felt which enclosed a horizontal cylindrical combustion chamber. The ceramic felt failed when flows rates exceeded 630 SCFH. Future design goals included improving the characteristics of the felt emitter to reduce pressure and temperature drop, evaluating long term strength and emittance, and optimizing the combustion and recuperation process to improve efficiency.
U.S. Pat. Nos. 5,383,976 and 5,439,532 to Fraas et al. disclose various gas-fired TPV electric generators employing SiC emitters. A variation on the SiC emitter tube design known as a "spine disc burner/emitter" is described by Fraas et al. in "SiC IR Emitter Design for Thermophotovoltaic Generators" (The Second NREL Conference on Thermophotovoltaic Generation of Electricity, Colorado Springs, Colo., 1995, pp. 488-494) which improves the conversion efficiency of chemical energy to emitter radiance. See also Fraas, et al. "Development of a Small Air-Cooled Midnight Sun Thermophotovoltaic Electric Generator" (The Second NREL Conference on Thermophotovoltaic Generation of Electricity, Colorado Springs, Colo., 1995, pp. 128-133), which describes a TPV system that generates a power output of 137 Watts.
Schroeder et al. in "An Experimental Investigation of Hybrid Kerosene Burner Configurations for TPV Applications" (The First NREL Conference on Thermophotovoltaic Generation of Electricity, Copper Mountain, Colo., 1994, pp. 106-118), describes laboratory investigations which coupled flow-through emitters to liquid fired burners. The method of atomization for this low fuel input burner (&lt;1 kg/hr) utilized ultrasonics. As described by Menchen in "Development of a 0.1 kW Thermoelectric Power Generator for Military Applications" (American Chemical Society, 1986, pp. 1361-1366) and by McAlonan et al. in "Burner System for a Thermoelectric Generator" (American Institute of Aeronautics and Astronautics, Inc., 1987, pp. 1962-1968), this atomization approach was used earlier in the development of small portable thermoelectric power systems for the U.S. Army.
It is thus clear that further improvements in TPV energy conversion efficiency and energy power density are needed before TPV electric power generation can be brought out of the laboratory and put to practical, commercial use. The present invention provides improvements to the burner/emitter/recuperator aspects of TPV systems.